Commercially and clinically, hydrogel contact lenses, including silicone hydrogel contact lenses, currently dominate the contact lens market. The maturation of the hydrogel lens market increases the pressure on lens manufacturers to increase quality while reducing cost.
Some documents describing silicone hydrogel contact lenses include: U.S. Pat. No. 4,711,943, U.S. Pat. No. 5,712,327, U.S. Pat. No. 5,760,100, U.S. Pat. No. 7,825,170, U.S. Pat. No. 6,867,245, US20060063852, US20070296914, U.S. Pat. No. 7,572,841, US20090299022, US20090234089, and US20100249356, each of which is incorporated in its entirety by reference herein.
In free radical polymerization of polymerizable compositions, reaction inhibition can occur as a result of the presence of oxygen, either in the form of dissolved oxygen gas present in the polymerizable composition, or in the form of oxygen gas present in the vapor space surrounding the mold before or during the curing process. Nitrogen purging and/or the use of vacuum conditions for removing unwanted oxygen from the polymerizable composition, from the mold cavity, and/or from the curing oven can be used to keep oxygen levels low before and during the curing process. However, the use of nitrogen purging and vacuum conditions can add significantly increase the cost of the manufacturing process, of the manufacturing equipment, and thus of the final lens product.
Additionally, regardless of the type of atmosphere present during filling and curing, many polymerizable compositions do not result in hydrogel contact lens that are ophthalmically acceptable as the lenses formed from these polymerizable compositions do not adequately retain their molded shape after hydration or after autoclaving. In other words, many polymerizable compositions produce contact lenses having undesirable characteristics such as being discolored, misshaped or distorted, or not retaining their molded shapes, etc., even when the polymerizable compositions are prepared and cured under low oxygen conditions or an inert atmosphere. Thus, there continues to be a need for new hydrogel contact lens formulations and manufacturing methods, particularly lens formulations that do not require expensive inert atmospheres to be provided during curing, or that use inexpensive ingredients to improve lens shelf life, reduce lens distortion, or improve lens shape retention.